The present invention relates generally to a regulator for RF amplifiers used in variable frequency applications.
One commonly known fact about most microwave RF transistors is they usually have more gain at lower frequencies than at higher frequencies. Therefore, microwave transistors used in variable frequency applications will vary in gain as their operating frequency changes from the low end of the frequency band to the high end of the frequency band. For applications that require performance over a wide frequency band, this can be a significant problem. For example, frequency variation in transistor gain can be extremely problematic for high stability applications that require a flat (frequency independent) gain response, such as for Moving Target Indicator (MTI) pulse-type radar systems.
Because of this difficulty, a very important parameter in the design of any microwave preamplifier/amplifier assembly used in variable frequency applications is the amount of allowable gain difference across the frequency band. One approach to solving this problem is to specify a maximum allowable gain difference for the transistors (for example, 1 dB across the frequency band). The parts are then screened to that specification. However, this approach can be extremely expensive, especially in large part count devices where there may be many RF amplifiers in parallel in a circuit. This approach also has the disadvantage that it may preclude the use of commercial-off-the-shelf (so-called xe2x80x9cCOTSxe2x80x9d) components altogether. This can greatly increase manufacturing costs to the point of impracticality.
Another approach is to simply relax the power out delta requirements across the band and use COTS transistors. However, this will degrade the performance of the amplifier and system across the operating band.
If COTS transistors are used, a conventional linear regulator can be used to regulate the voltage provided to the preamplifier/amplifier. In a typical linear regulator, the voltage is preset to a predetermined voltage (xe2x80x9creference voltagexe2x80x9d) for normal operation. In some cases this voltage is tunable so that a more accurate voltage can be set when the load is applied. Generally, however, once the regulator output voltage is set it does not change during normal operation. Accordingly, a conventional linear regulator used with COTS transistors will lead to the same problems discussed above regarding the variation of the system output response with frequency. In high stability, frequency-variable applications, like MTI radar, this is a significant drawback that degrades system performance.
Additionally, a linear regulator output voltage will tend to decrease in value during load pulses due to the xe2x80x9cdroopxe2x80x9d in the output capacitor voltage. This droop in the output voltage causes a slight decrease in RF transistor power output, thereby adding to (exacerbating) an already decreasing output power as frequency increases.
FIG. 1 (not to scale) generally illustrates the difficulty posed in the prior art. At microwave frequencies, a typical transistor gain falls off at the higher frequencies, while the linear regulator voltage output remains more or less constant. The net effect at the system level is that the system response at higher frequencies will be significantly less than at the lower frequencies. FIG. 1 illustrates the problem in one context (microwave frequency performance of transistors).
It can readily be appreciated that the problem exists in other contexts. For example, instead of a transistor, there may be some other component, device, or system, whose output response or gain exhibits some variation with frequency. Also, the variation of this output response or gain may increase with frequency, decrease with frequency, or increase and decrease with frequency at different points in the spectrum. Finally, the frequency span of interest can be other than microwave frequencies. The common problem is that of how to control, or compensate for, the frequency variable output response in order to render the desired effective response. Generally, the desired effective response is flat over an operational frequency band for the system at issue. Sometimes, a non-flat response may be desired.
Other problems and drawbacks also exist.
An embodiment of the present invention comprises a power regulator for regulating the power provided to a transistor-based circuit, such as that which might be employed in a high stability MTI radar system. According to the invention, a variable reference voltage is controlled based on the frequency of operation. The output voltage of the power regulator is then modified accordingly. Preferably, the reference voltage is controlled based on the location of the target frequency in relation to the overall operational frequency band, which has been divided into a series of sub-bands.
According to another aspect of the invention, a power regulator is provided having an input circuit for providing inputs pertaining to frequency; a control circuit for determining a modified reference voltage or voltage offset; an error amplifier or differential amplifier for modifying the output voltage; a pass-through circuit for passing the output voltage; and a discharge circuit for discharging the output voltage.
According to another aspect of the invention, methods are provided for regulating power provided to a transistor-based circuit. According to one embodiment, the operational frequency band is divided up into a series of sub-bands. The power regulator output voltage is then controlled based on a specific (target) frequency. The output response of the transistor-based circuit is maintained as substantially uniform over frequency. According to another embodiment, input values are provided that correspond to a target frequency. A modified output voltage is computed that is appropriate for the target frequency. A modified output voltage for the voltage regulator is computed, and the reference voltage is modified accordingly. The output response is maintained as substantially independent of transistor gain variations across frequency.
Accordingly, it is one object of the present invention to overcome one or more of the aforementioned and other limitations of existing systems and methods for addressing gain variations in frequency variable circuits.
It is another object of the invention to provide a power regulator that solves or mitigates the problem of the variable frequency response of electronic circuits, such as transistor-based circuits.
It is another object of the invention to provide a power regulator that solves or mitigates the problem of the variable frequency response in transistor-based circuits so that COTS transistors can be used without special screening procedures.
It is another object of the invention to provide a power regulator for use in high stability radar systems so that the system response over the frequency range of interest remains substantially constant.
It is another object of the invention to provide methods for regulating power so that the power provided to variable frequency response circuits compensates for that frequency-dependence so as to render an overall substantially flat system frequency response.
The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute part of this specification, illustrate several embodiments of the invention and, together with the description, serve to explain the principles of the invention. It will become apparent from the drawings and detailed description that other objects, advantages and benefits of the invention also exist.